1. Field of the Invention
This invention relates to improvements in a combustion control system for a diesel engine, and more particularly to the combustion control system for effecting a low temperature premix combustion in a combustion chamber of the engine so as to improve exhaust emission performance and combustion noise performance of the engine.
2. Description of the Prior Art
Hitherto a variety of techniques for improving exhaust gas performance and combustion noise performance of a diesel engine has been proposed and put into practical use. One of them is disclosed in Japanese Patent Provisional Publication No. 8-86251 and configured as follows: A fuel injection timing is retarded to a timing after top dead center on compression stroke thereby prolonging a so-called ignition delay period of between a fuel injection initiation timing and an ignition timing of injected fuel. During this prolonged ignition delay period, a pre-mixture of air and fuel is formed in the combustion chamber by lowering an oxygen concentration owing to exhaust gas recirculation and by controlling gas flow in the combustion chamber, thus accomplishing a so-called low temperature premix combustion.
Now, the low temperature premix combustion aims at the fact that fuel is dispersed around oxygen as much as possible before initiation of ignition. In this regard, the latest experiments have revealed that the most important condition for effectively realizing the low temperature premix combustion is to terminate fuel injection during the ignition delay period (i.e., the ignition delay period is longer than a fuel injection period for which fuel is being injected), in addition to promoted dispersion of fuel under a gas flow control. Accordingly, if the ignition delay period is changed owing to difference in circumferential factors such as difference in cetane number of fuel so as to become shorter than the fuel injection period, the low temperature premix combustion cannot be accomplished and therefore exhaust gas purification performance inherent in the low temperature premix combustion cannot be obtained. For example, in case that the cetane number of fuel is high so that the ignition delay period is shorter than the fuel injection period, the low temperature premix combustion cannot be realized and therefore diffusive combustion is mainly made in the combustion stroke thereby increasing smoke in exhaust gas.
In case that the cetane number of fuel is low, the ignition delay period is prolonged and therefore such a low cetane number is preferable for the low temperature premix combustion. However, if the ignition timing of fuel is further retarded owing to use of low cetane number fuel upon having been originally retarded in the low temperature premix combustion, fuel economy is degraded owing to lowering in combustion pressure in a high EGR rate operating range while causing misfire thereby increasing unburned fuel emission. Additionally, this increases combustion noise in a low EGR rate operating range of the diesel engine.
An object of the present invention is to provide an improved combustion control system for a diesel engine, which can overcome drawbacks encountered in conventional combustion control systems for diesel engines.
Another object of the present invention is to provide an improved combustion control system for a diesel engine, which can effectively prevent performances of smoke, fuel economy and combustion noise from being degraded under the influence of circumferential factors such as cetane number of fuel.
A further object of the present invention is to provide an improved combustion control system for a diesel engine, by which an ignition delay period is controlled to be longer than a fuel injection period thereby effectively accomplishing low temperature premix combustion.
A still further object of the present invention is to provide an improved combustion control system for a diesel engine, which can accomplish such a control as to cause the relationship between an ignition delay period and a fuel injection period to fall into a predetermined state when an ignition timing shifts from a target value.
A first aspect of the present invention resides in a combustion control system for a diesel engine, comprising a section for changing an ignition timing. A section is provided for changing a fuel injection period. A section is provided for detecting the ignition timing. Additionally, a section is provided for controlling a relationship between an ignition delay period and the fuel injection period to fall into a predetermined state when the detected ignition timing shifts from a target value. Accordingly, even if the ignition timing is changed owing to circumferential factors such as cetane number of fuel, smoke in exhaust gas, fuel consumption and combustion noise can be effectively prevented from being degraded.
A second aspect of the present invention resides, as shown in FIG. 14, in a combustion control system for a diesel engine, comprising a section 81 for detecting an ignition timing. Additionally, a section 82 is provided for controlling the detected ignition timing to generally coincide with or be retarded relative to a fuel injection termination timing in a predetermined engine operating region. Accordingly, the ignition timing is necessarily controlled to generally coincide with or retarded relative to the fuel injection termination timing. As result, the ignition delay period is controlled to be longer than the fuel injection period, thereby effectively accomplishing the low temperature premix combustion. This can effectively prevent smoke in exhaust gas and combustion noise from increasing even if circumferential factors such as cetane number of fuel are changed.
A third aspect of the present invention resides, as shown in FIG. 15, in a combustion control system for a diesel engine, comprising a section 81 for detecting an ignition timing. A section 91 is provided for judging that the detected ignition timing is retarded relative to a target value. A section 92 is provided for controlling the detected ignition timing so as to be restored to the target value when the detected ignition timing is retarded relative to the target value. Accordingly, the relationship between the ignition delay period and the fuel injection period can be restored to the target state even in case that the ignition timing is changed owing to low cetane number fuel or the like in a non-low temperature premix combustion region, thereby preventing combustion noise from increasing while suppressing smoke in exhaust gas.
A fourth aspect of the present invention resides, as shown in FIG. 16, in a combustion control system for diesel engine, comprising a section 81 for detecting an ignition timing. A section 91 is provided for judging that the detected ignition timing is retarded relative to a target value. Additionally, a section 101 is provided for controlling an oxygen concentration in a combustion chamber so as to be decreased to a predetermined value when the detected ignition timing is retarded relative to the target value. Accordingly, a control point can be shifted into a region equivalent to that contains a target value even in case that it is difficult that the relationship between the fuel injection period and the ignition delay period cannot be restored to the target state in the non-low temperature premix combustion region, owing to an excessive prolongation of the ignition delay period, for example, due to low cetane number fuel. This effectively achieves suppressing combustion noise of the engine.
A fifth aspect of the present invention resides, as shown in FIG. 17, in a combustion control system for a diesel engine, comprising a section 81 for detecting an ignition timing. A section 111 is provided for judging that first and second conditions are established in a predetermined engine operating region. The first condition is established when the detected ignition timing is advanced relative to a fuel injection termination timing. The second condition is established when it is difficult to control the detected ignition timing to generally coincide with or retarded relative to the fuel injection termination timing. Additionally, a section 112 is provided for controlling an amount of oxygen in a combustion chamber of the engine to increase when the first and second conditions are established. Accordingly, the oxygen amount in the combustion chamber can be increased in case that it is difficult to control the ignition timing to generally coincide with or retarded relative to the fuel injection termination timing under a condition where the actual ignition timing is advanced relative to the fuel injection termination timing. This can effectively reduce smoke in exhaust gas.